BUG: Incorrect index shape when using a user-defined function for aggregating a grouped series with object-typed index.

pandas/core/groupby/ops.py

@@ -738,7 +738,7 @@ def agg_series(self, obj: Series, func: F):
             # TODO: can we get a performant workaround for EAs backed by ndarray?
             return self._aggregate_series_pure_python(obj, func)
 
-        elif obj.index._has_complex_internals:
+        elif obj.index._has_complex_internals or obj.index.dtype == "object":
             # Preempt TypeError in _aggregate_series_fast
             return self._aggregate_series_pure_python(obj, func)
 

pandas/tests/groupby/test_function.py

@@ -1087,3 +1087,11 @@ def test_groupby_sum_below_mincount_nullable_integer():
     result = grouped.sum(min_count=2)
     expected = DataFrame({"b": [pd.NA] * 3, "c": [pd.NA] * 3}, dtype="Int64", index=idx)
     tm.assert_frame_equal(result, expected)
+
+
+def test_groupby_index_object_dtype():
+    # GH 40014
+    df = DataFrame({"c0": ["x", "x", "x"], "c1": ["x", "x", "y"], "p": [0, 1, 2]})
+    df.index = df.index.astype("O")
+    grouped = df.groupby(["c0", "c1"])
+    grouped.p.agg(lambda x: all(x > 0))